1. Field of the Invention
The present invention relates to a production method of Nb3Al superconducting multifilamentary wire and more particularly to a production method of Nb3Al superconducting multifilamentary wire by the rapid-heating, quenching and transformation(RHQT) processing, capable of improving critical temperature Tc, upper critical field Bc2 and critical current density Jc.
2. Description of the Related Art
As compared to an ordinary superconducting wire such as Nb3Sn and NbTi, the RHQT Nb3Al superconducting multifilamentary wire has more excellent critical current density characteristic and strain tolerance characteristic in a high magnetic field. Thus, it is expected that this wire will be applied to a large-scale superconducting applications, such as a fusion reactor and a high-energy accelerator, in which a large electromagnetic force is applied to a superconducting wire thereof itself.
Conventionally, after heating the Nb/Al multifilamentary composite wire produced by the Jerry Roll (JR) method or Rod-in-Tube (RIT) method up to Nb(Al) bcc solid solution region at about 1,900xc2x0 C. quickly, it is quenched to produce a composite wire, in which supersaturated solid solution Nb(Al)ss filament having Nb-25 at % Al composition is dispersed in Nb matrix thereof, and this Nb(Al)ss is transformed at a temperature of 700-800xc2x0 C. to produce Nb3Al superconducting multifilamentary wire. In such Nb3Al superconducting multifilamentary wire, crystal grain of A15 type Nb3Al generated by transformation is as small as several tens nm and this grain boundary serves as the center of main pinning for fluxoid, so that Jc is extremely high.
Further, for the Nb3Al superconducting multifilamentary wire, external stabilization method of applying Cu foil as a stabilizer by mechanical clad processing after quenching has been developed using the fact that a supersaturated solid solution has an excellent deformability at room temperature. This Cu clad processing has an advantage that deformation of the saturated solid solution improves Jc after transformation about twice.
However, by the conventional transformation heat treatment method for production of Nb3Al superconducting multifilamentary wire, Tc of the resultant Nb3Al compound is 17.8 K and Bcz (4.2 K) at a middle point of a resistance transition curve is 26 T. If the deformation amount in clad processing exceeds 40% in terms of section reduction ratio, Jc begins to deteriorate. Then, in deformation at about 40%, sufficient adhesion cannot be secured between Cu and the quenched wire, so that an electric resistance of interface is so high that the Cu has not acted enough as the stabilizer.
On the other hand, in order to enhance Tc more than 18.3 K or Bcz (4.2 K) more than 29 T, it has been found effective to heat the Jerry-Roll (JR) or Rod-in-Tube (RIT) Nb/Al composite multifilamentary wire to generate disorder A15 type Nb3Al phase in it by direct diffusion at a high temperature of 1700-1900xc2x0 C. and subsequentry carry out the second stage heat treatment to improve the long range ordering at 700-800xc2x0 C.
However, in this case, the Cu stabilizer cannot be applied by clad processing because the resulted wire is mechanically brittle and further, the crystal grain of Nb3Al becomes coarse. Thus, there is such a defect that the Jc at a low magnetic field is significantly deteriorated.
Accordingly, the present invention intends to solve the above-described problems of the conventional art and provide a production method for a high-performance Nb3Al superconducting multifilamentary wire based on rapid-heating, quenching and transformation method, in which critical temperature, upper critical magnetic field and critical current density are improved.
To achieve the above-described object, according to a first aspect of the present invention, there is provided a production method for Nb3Al superconducting multifilamentary wire based on rapid-heating and quenching method, wherein upon a first stage heat treatment of heating a composite, in which bcc phase Nbxe2x80x94Al supersaturated solid solution is dispersed in Nb matrix, the bcc phase Nbxe2x80x94Al supersaturated solid solution ordered in temperature rise process is made disorder at an initial phase thereof, a non-reacting portion located adjacent is heated using a reaction heat generated when transforming this disordered bcc phase to A15 phase, disorder of the bcc phase is promoted while propagating a high-temperature transformation region to automatically progress high-temperature heat treatment thereby causing a transformation to block stacking fault from being generated in the A15 phase and crystal grain from being coarse and then, a second stage heat treatment for improving a long range ordering of the A15 phase is carried out.
According to a second aspect of the present invention, there is provided a production method for Nb3Al superconducting multifilamentary wire according to the first aspect wherein the temperature of the first stage heat treatment is 850-1,100xc2x0 C. and holding time thereof is 1 second-1 hour.
According to a third aspect of the present invention, there is provided a production method for Nb3Al superconducting multifilamentary wire according to the first or second aspect wherein the temperature of the second stage heat treatment is 650-800xc2x0 C. and holding time thereof is 3-200 hours.
According to a fourth aspect of the present invention, there is provided a production method for Nb3Al superconducting multifilamentary wire according to the first-third aspect wherein volume ratio of bcc phase Nbxe2x80x94Al supersaturated solid solution with respect to Nb matrix is 0.1-3.
According to a fifth aspect of the present invention, there is provided a production method for Nb3Al superconducting multifilamentary wire according to the first-fourth aspect wherein the bcc phase Nbxe2x80x94Al supersaturated solid solution is subjected to mechanical deformation of 1-90% in terms of section reduction ratio.
According to a sixth aspect of the present invention, there is provided a production method for Nb3Al superconducting multifilamentary wire according to the first-fifth aspect wherein Cu is applied to the surface of a composit, in which the bcc phase Nbxe2x80x94Al supersaturated solid solution is dispersed in Nb matrix, as a stabilizer by clad processing or electric plating.
According to a seventh aspect of the present invention, there is provided a production method for Nb3Al superconducting multifilamentary wire according to the first-sixth aspect wherein Ag or Cu as a stabilizer is isolated with Nb diffusion barrier from the bcc phase Nbxe2x80x94Al supersaturated solid solution within the composite, in which the bcc phase Nbxe2x80x94Al supersaturated solid solution is dispersed in the Nb matrix.
According to an eighth aspect of the present invention, there is provided a production method for Nb3Al superconducting multifilamentary wire according to the first-seventh aspect wherein when by adding an element M to the bcc phase Nbxe2x80x94Al supersaturated solid solution by alloying, composition thereof is expressed with Nby (Al1-xMx)1-y, if the added element is Ge, it is 0.05-0.2 while if the added element is Si, it is 0.05-0.15.
According to a ninth aspect of the present invention, there is provided a production method for Nb3Al superconducting multifilamentary wire according to the first-eighth aspect wherein a composit in which the bcc phase Nbxe2x80x94Al supersaturated solid solution is dispersed in the Nb matrix is wound in the form of coil.
The present invention has been achieved based on knowledge of the inventor as follow.
That is, the inventor of the present invention has made efforts for optimizing transformation technology for Nb3Al wire to be produced according to the rapid-heating, quenching and transformation method. As a result, it has been found that under the conventional transformation method, order reaction of a supersaturated solid solution and transformation reaction to the A15 phase are progressed in competition with each other. Further, the superconducting characteristic of Nb3Al compound maybe deteriorated by transformation from such ordered bcc phase. If the bcc phase is ordered, the A15 phase ordered to some extent from the beginning is transformed and it is estimated that the A15 phase generated in such a manner contains a large amount of stacking faults. The inventor estimated that if the supersaturated solid solution ordered in temperature rise process was made disorder again, deterioration of the superconducting characteristic could be suppressed. Then, he tried making disorder the supersaturated solid solution and transformation from that state by heating up to a predetermined temperature, which is higher than conventionally, namely, 850xc2x0 C.-1,100xc2x0 C., preferably 900xc2x0 C.-1,050xc2x0 C. As a result, it was found that (1) such heat treatment method did not order the supersaturated solid solution just until the transformation was started, (2) the temperature of the specimen was raised from several tens xc2x0 C. to several hundreds xc2x0 C. by a reaction heat of the transformation, (3) nucleated transformation was propagated throughout the specimen immediately, and (4) if the transformation was completed, the specimen temperature turned down to a predetermined temperature before the transformation immediately.
The present invention provides a new two-stage heat treatment method using this phenomenon (reaction-transformation).